when there is a couple and when they are together they are going out, but when they are alone they are counted as single leaving them free to do as will.
Therefor it is when you and a girl/boy are not officially a couple but you are more then friends with benefits
guy 1 - dude, don't hook up with her you have your girlfriend!
guy 2 - narr mate she's not my girlfriend we are just 'single going out' and shes not here so i can do what i want with who i want.
guy 1 - ohok mad bro, GO FOR IT!